The invention relates generally to integrated circuit fabrication processes and manufacturing methods and more particularly to copper metallization techniques used in the formation of conductive interconnections in integrated circuits.
The use of copper for interconnects in integrated circuits is an important field of research for integrated circuit manufacturers. As individual features on integrated circuits (ICs) become smaller, the size of metallized interconnects (lines, vias, etc.) also shrinks. The reduced size of interconnects can create unacceptable resistance in aluminum or tungsten conductors which increases impedance and propagation delays and can limit microprocessor clock speeds. Aluminum also is susceptible to electromigration which, in very fine (i.e., small cross-section) conductors, can cause discontinuities which produce device failure. Copper""s greater conductivity, when compared with aluminum, tungsten, or other conductive materials used in ICs, is an important advantage. Copper also has greater resistance to electromigration. Both factors are important to manufacturers of ultra-large-scale-integration (ULSI) IC circuits, which is why copper is the subject of intensive research. The conductivity of copper is approximately twice that of aluminum and over three times that of tungsten. Copper thus is clearly advantageous for use in devices with ever-smaller geometries. With respect to electromigration, copper is approximately ten times better than aluminum, meaning that copper will better maintain electrical integrity.
A principle disadvantage of copper, presenting numerous processing problems for IC manufacturers, is its polluting effect on semiconductor materials. When copper comes in contact with semiconductor material it changes the semiconductor characteristics and destroys active circuit devices. A solution to this problem is to deposit a diffusion barrier material on the IC in regions where contact with copper metal must be avoided. The barrier material blocks the migration of copper into critical semiconductor regions while permitting electrical communication between the copper and the regions of the IC underlying the barrier material. TiN and TaN are examples of well-known and widely used diffusion barrier materials employed in IC copper metallization processes.
But diffusion barriers present another problem associated with copper metallization, copper adheres poorly to diffusion barrier materials. One prior art approach to adhering copper to diffusion barrier materials is to deposit the copper by means of physical vapor deposition (PVD), alternatively referred to as sputtering. PVD involves directing metallic copper onto a target surface. PVD improves adhesion between and the barrier material, but copper deposited by PVD exhibits poor conformality with surface features such as steps and high-aspect-ratio vias and trenches.
An alternative copper deposition process is chemical vapor deposition (CVD). In CVD, copper is combined with a ligand, or organic compound (the combination is called a copper precursor), and volatilized. The IC wafer or substrate, coated with diffusion barrier material, is heated and exposed to the precursor which decomposes when it strikes the copper-receiving surface. The heat drives off the organic material and leaves copper behind. Copper applied by conventional prior art CVD processes has greater conformality to surface features than copper deposited by PVD. But for most precursors, CVD adheres poorly to diffusion barrier materials.
Various techniques and been devised to improve the adhesion of CVD copper to barrier material. A typical approach is to apply CVD copper immediately after the deposition of the diffusion barrier material. It has generally been thought that the copper layer has the best chance of adhering to the diffusion barrier material when the diffusion barrier material surface is clean. Hence, the diffusion barrier surface is often kept in a vacuum, or controlled environment, and the copper is deposited on the diffusion barrier as quickly as possible. However, even when copper is immediately applied to the diffusion barrier surface, problems remain in keeping the copper properly adhered. A complete understanding of why copper does not always adhere directly to a diffusion barrier surface is lacking.
Despite the large amount of effort that has been expended on CVD, two major obstacles remain before a CVD copper process can be adopted in manufacturing. These two critical hurdles are (i) high cost of ownership (COO) for the CVD process and (ii) reliable adhesion to barriers. The presently available MOCVD processes and precursors do not satisfactorily fulfill both these criteria simultaneously without compromising film and process attributes. Since the precursor cost is a major contributor ( greater than 65%) to the overall COO of the CVD process, precursors that can be inexpensively manufactured are preferred. However, precursor costs have to be lowered without compromising film properties. For instance, reliable and repeatable adhesion has to be achieved while simultaneously maintaining low via and contact resistance low, high deposition rate, high conformality as well as low cost of the precursor. Many IC manufacturers have employed a PVD Cu seed layer followed by a CVD Cu fill in order to achieve adequate film properties. The use of a PVD Cu seed layer underscores the difficulty in achieving low contact resistance and reliable adhesion on barriers (TiN or TaN) by a CVD process alone.
As the size of features on ICs continues to shrink, it is desirable to continue developing improvements in the adhesion of CVD copper to barrier materials as a replacement for PVD, which is unsuitable for metallizing the smallest features.
It would be advantageous to provide a method of improving the adherence of copper metallization to diffusion barrier material without the sacrifice in conformality associated with PVD copper.
It would also be advantageous to provide a method of depositing copper on diffusion barrier material using chemical vapor deposition (CVD) to improve conformality, while also improving the adhesion between the copper and the barrier material.
In addition, it would be advantageous to discover a method adhering a thin seed layer of copper to surfaces of diffusion barrier material using high conformality CVD, wherein the thin seed layer serves as a receiving surface for the deposition of additional copper deposited by CVD using the most cost-effective precursors, with the second layer of CVD-deposited copper adhering strongly to the seed layer of copper through copper-to-copper bonds.
Accordingly, a method is provided for use in integrated circuit manufacturing for applying copper to copper-receiving surfaces of an integrated circuit substrate. The method comprises steps which include positioning the integrated circuit substrate in a chemical vapor deposition (CVD) chamber. In the CVD chamber, a first layer of copper is deposited on the copper receiving surfaces by means of chemical vapor deposition. The first layer of copper conforms and adheres to the copper-receiving surface and provides a copper layer to which subsequently-deposited copper will adhere. And then a second layer of copper is deposited by means of CVD on the first layer until a predetermined thickness of copper is formed on each copper receiving surface.
In the preferred embodiment of the present invention, the first layer of copper is deposited by means of CVD using (hfac)Cu(1,5-Dimethylcyclooctadiene) precursor, which has been found to exhibit good conformality and adhesion under a wide range of processing conditions.
The first adhering conforming layer of copper deposited, regardless of the precursor used, is deposited to a maximum thickness of 1000 angstroms, with a suggested thickness range generally in the range of 50 to 300 angstroms and a preferred thickness generally in the range of 100 angstroms to 200 angstroms.
In the preferred embodiment of the invention the second layer of CVD copper is deposited using a different precursor from that used in depositing the first adhering conforming layer of CVD copper. If the suggested precursor, (hfac)Cu(1,5-Dimethylcyclooctadiene), is used in the deposition of the first layer of copper, the method recommends using a precursor other than (hfac)Cu(1,5-Dimethylcyclooctadiene) for depositing the second layer. The step of depositing a second layer of copper is preferably carried out in the same CVD chamber in which the first layer of copper is deposited using, although use of the different CVD chambers to deposit the first and second copper layers is within the scope of the invention.
The copper-receiving surfaces of an integrated circuit on which copper is deposited in accordance with the present invention will generally be diffusion barrier material applied to the integrated circuit substrate. Accordingly, an embodiment of the present invention preferably includes the preliminary step of depositing diffusion barrier material on the integrated circuit to form the copper-receiving surfaces on which the above-described method is practiced. The step of depositing diffusion barrier material includes depositing material selected from the group consisting of TiN, TiON, TiSiN, Ta, TaSiN, TaN, TiW, TiWN, Mo, WN, and WSiN. Deposition of the diffusion barrier material can be performed by means of CVD or PVD. If CVD deposition of the diffusion barrier material is employed, the first, or the second, or both layers of CVD copper can be deposited using the same CVD chamber used in the deposition of the diffusion barrier material.
The method of the present invention is alternatively characterized as a method of applying a seed layer of copper on diffusion barrier material in integrated circuit manufacturing. The seed layer of copper is provided as a layer to which additional copper, subsequently deposited, will adhere. The method comprises positioning an integrated circuit which has diffusion barrier material deposited thereon in a chemical vapor deposition (CVD) chamber. A thin layer of copper is then deposited by means of CVD on the diffusion barrier material. The preferred precursor for the CVD of the thin layer of copper is (hfac)Cu(1,5-Dimethylcyclooctadiene). Once the seed layer of copper has been deposited, the integrated circuit substrate is ready to receive a much thicker xe2x80x9cfillxe2x80x9d or xe2x80x9cbulkxe2x80x9d (second) layer of copper which will preferably be deposited on the substrate by CVD using a precursor other than (hfac)Cu(1,5-Dimethylcyclooctadiene). The terms xe2x80x9cfillxe2x80x9d and xe2x80x9cbulkxe2x80x9d as used herein means the second layer of CVD copper is intended to fill the interiors of vias and trenches and cover other topographical features on the surface of the substrate, and also means the second layer represents the majority of the mass of copper deposited on the substrate. The method assures that all the CVD deposited copper will strongly adhere to the diffusion barrier material since the seed layer is deposited using a precursor selected to provide good adhesion to barrier material and the second layer will adhere to the seed layer due to the mechanical strength of the copper-to-copper bonding which occurs between the copper layers.